In the formation of silver halide emulsions suitable for use in photographic materials, it is necessary to sensitize the emulsions. Chemical sensitization is utilized to improve the photo efficiency of the emulsions. Spectral sensitization is utilized to make grains sensitive to specific wavelengths of light. The addition of chemical and spectral sensitizing materials to silver halide grains normally is referred to as finishing of the grains. During finishing, other additives are also introduced into the emulsions, such as antifoggants, coating aids, ripeners, supersensitizers, and surfactants.
The application of heat during emulsion finishing has a tendency to raise the minimum fog level of unexposed areas. Fog also may increase in an emulsion during storage. Therefore, the use of antifoggants is necessary to minimize these effects. Such antifoggants are discussed in Research Disclosure 36544 of September 1994 in Section VII.
The antifoggants generally are added during the finishing process after chemical sensitization and prior to, during, or after the spectral sensitization. There is a continuing need for improvements in the efficiency of antifoggants.
It is known in the formation of high chloride grains (above 90%) to utilize bromide as a material added during finishing. It is added to the grain surface in order to improve the adsorption of sensitizing dyes onto the grain surface, enhance the speed/fog performance of the grains, and also improve reciprocity. Generally this material is added as a sodium or potassium bromide salt. It is also known that bromide may be added to the emulsion by the addition of a Lippmann (fine grain) emulsion to the finish. Such a process is illustrated in U.S. Pat. No. 4,865,962. Other photographic materials may be with a fine grain emulsion as shown in Konica JP 02-103,032 (1990).
Generally modern negative-working color photographic paper utilizes high chloride emulsions. Such emulsions, while allowing rapid development and high quality images, are subject to fog upon storage.
Problem to be Solved by the Invention
There is a continuing need for improvements in Dmin of negative-working photographic papers by decreasing the fog. Particularly, there is a need to prevent the increase of fog during storage of such papers prior to use. There is also a need to more efficiently use known antifoggants.